Comparison of the patterns of expression of specific enzymes in closely related species of Hawaiian Drosophila have revealed several cases of striking differences that reflect differences in regulatory genes. Some of these differences are apparently qualitative rather than quantitative. For example, a particular tissue of one species lacks detectable activity of an enzyme present at a high level in that enzyme in one or more other tissues. Analysis in hybrids has demonstrated both cis- and trans-acting factors with such dramatic regulatory effects. We are investigating the molecular basis of some of these regulatory differences. Kinetics of enzyme synthesis and degradation will be compared using isotopic labeling methods. Specific nucleic acid hybridization probes will be obtained, using homologous genes already cloned from D. melanogaster where possible, and used to investigate messenger levels, RNA processing, transcriptional activity, etc. Finally, in the case of cis-acting regulators, sequence organization around the affected structural genes will be investigated using physical-chemical mapping procedures (restriction mapping, R-looping, etc.) in hopes of identifying sequences relevant to the regulatory differences.